1. Field of the Invention
This invention pertains to wide bandgap electronic devices, such as high frequency and high power devices; optoelectronic devices, such as visible light emitters to ultraviolet lasers and detectors; and a method for making such devices.
2. Description of Related Art
Wide bandgap semiconductor nitrides have demonstrated and continue to hold significant promise in a wide range of device technologies. This family of semiconductors has a tunable direct bandgap that ranges from 8 eV up to 6.2 eV, covering the IR, visible, and UV portions of the electromagnetic spectrum making them well suited to optoelectronic applications from visible light emitters to UV lasers and detectors. It is this application area that has received the most commercial attention to date, resulting in billion dollar LED industries. Perhaps the largest promise from wide bandgap semiconductor nitrides is yet to be fulfilled, that being in high frequency and high power electronics. The high breakdown fields and moderate-to-high electron mobilities in these materials make them ideal for GHz transistors and high voltage/high current power handling devices. Both areas are of critical need to future systems, such as wide bandwidth communications and control, and the all-electric ship.
Despite this considerable promise, a fundamental hurdle has limited progress in realizing some of these device technologies. That hurdle is a lack of native substrate that would permit homoepitaxy. Instead, wide bandgap semiconductor nitride films are grown heteroepitaxially on sapphire, silicon carbide, or other substrates. The lattice parameter and thermal coefficient of expansion difference between host substrate and nitride thin film result in the creation of stress and stress-relieving dislocations. These dislocations, or extended defects, propagate vertically in the film and can provide a vertical leakage path through layers grown for device applications. Further, these and related defects result in compensated films that make it difficult to controllably dope the material at low levels consistent with blocking layers in high power devices.
These challenges have stimulated research in novel methods of growing the material to reduce or eliminate the extended defects. These defect reduction efforts can be categorized into two techniques: epitaxial lateral overgrowth and growth on etch delineated surfaces. Epitaxial lateral overgrowth involves the masking of a continuous III-V nitride surface with either a silicon dioxide or a silicon nitride mask, growing up through the openings and then laterally over the masked area. Low defect material for devices is found in the wings that grow over the masked area; hence devices must be placed in these select areas. Growth on etched substrate surfaces of sapphire and silicon carbide, inter alia, has also been used to reduce stress and extended defect densities. In this technique as well, only a small fraction of the grown material is useful and available for device fabrication. In both cases, the height variation of the wafer surface becomes considerable and it is necessary to fabricate devices on top of this topography, a challenge for any lithographic process. Such approaches are not suitable for large area power devices and are inefficient for any integrated device manufacturing technology.